Men who slept poorly had lower sperm counts and fewer sperm that had formed correctly, compared with men who slept better.

"Given the facts that approximately 20 percent of all young men may have reduced semen quality, and that sleep disturbances are common and increasing in industrialized countries, the results of this study may have important public health implications," the researchers wrote in their article.

The researchers used data from 953 young men who were mostly in their late teens and early 20s. They asked the men how well they had slept in the previous four weeks, conducted blood tests to measure their hormone levels and analyzed their semen.

The researchers found that 15 percent of the men said they had found it difficult to fall asleep, and 13 percent of the men reported sleeping restlessly.

In general, the worse that men slept, the poorer the quality of their semen was. For instance, the men who had slept the poorest had a 25 percent reduction in sperm count, and had 1.6 percent fewer sperm that were morphologically normal, compared with men who reported low levels of sleep disturbances. The researchers accounted for factors that could affect the results, such as men's alcohol consumption, smoking and age.

There were no differences in hormone levels across the groups, the researchers said.

The study found an association, and does not prove that there is a cause-and-effect relationship between sleep quality and sperm counts.

However, there are plausible ways to explain the link, the researchers said. It may be that sleep disturbances alter nighttime testosterone rhythms, without affecting overall testosterone levels, the researchers said. However, lifestyle factors not accounted for in the study could explain both the poor sleep and lowered semen quality, they noted.

The study was published online April 7 in the American Journal of Epidemiology.